After a borehole has been drilled into the ground, the casing cemented into place, and the formation completed so that production flows from the formation, up the tubing string, and to the surface of the ground, it is often desirable to backsurge the well in order to more thoroughly clean out some of the perforations, and unplug some perforations which may be clogged with debris. This operation is desirable in new completions which have been perforated, for example, with wireline casing guns, or on the other hand, it is often desirable to practice this backsurging technique on old producing wells that have been shut in or killed for some time, wherein the tubing often has been pulled and the well left dormant.
There are many occasions when a newly completed well does not produce at the rate anticipated by the logging engineer. Sometimes this lower production rate is due to some of the perforations being partially obscured or plugged.
In wells which have been produced for a considerable length of time, the perforations sometimes become plugged with accumulated debris or formation particles which have been translocated from the fluid-producing strata downstream towards the casing perforations.
It is also possible for the jet action of the shaped charges to form a densified wall or barrier between the perforations and the tunnels leading back up into the formation. In any event, proper backsurging techniques provide remedial action by which production rates can often be improved.
In order to properly backsurge a well, it is necessary to "shock" the payzone by the provision of a pressure differential suddenly effected between the perforations and the production tubing interior. A well which has been shut in at the surface until the bottom hole pressure increases to a suitable pressure, and then is open-flowed by opening the wellhead valve, will not provide proper backsurging for the reason that the volume of the production tubing acts to throttle or dampen the driving force across the perforations so that when the valve at the surface is suddenly opened, there is a relatively low change in flow rate which slowly increases to a maximum as the pressure differential from the perforations to the surface valve reaches equilibrium. This technique does not shock the well.
It is therefore desirable to backsurge a well with the control valve being located as close as possible to the perforations. This process provides a differential pressure between the tubing string and formation so as to cause a sudden flow through the perforations from the formation which is of a sufficient magnitude to translocate the debris in the tunnels and perforations into the tubing string, where the debris is produced along with the well fluids.
U.S. Pat. No. 3,662,834 to Young discloses a method and apparatus for cleaning previously perforated wells which are to be later fractured, acidized, or otherwise treated. The apparatus includes a valve series connected in the tubing string, with an elongated sleeve slidably mounted around the valve to open and enclose ports extending therethrough. Pressure is effected through the tubing string to move the sleeve downwardly and align ports to permit wellbore fluids from the formation to flow into the tubing string. A barrier is mounted within the valve to form a lower chamber which is preferably at atmospheric pressure whereby upon opening the valve, a sudden high pressure differential is developed between the connate fluids in the formation and the closed chamber. This sudden pressure differential across the perforated wellbore interval induces a rapid, high-velocity flow of connate fluids from the formation, through the perforations, and into the closed chamber. These rushing fluids wash out the debris, along with any loose formation materials, from the perforations. Once this sudden flow ceases upon the filling of the chamber, and preferably the loose materials are given time to settle into the lower portion of the chamber, the barrier is removed to permit the injection of fluid into the perforations for treating the well. The principle disadvantage is the pressurizing of the tubing string to a pressure greater than the formation pressure, i.e. overbalance, to actuate the valve, whereby the backsurge ceases upon the closed chamber being filled.
Apparatus and method for achieving the above-desired well techniques is the subject of the present invention.